Clock synchronization when switching between broadcast and data transmission modes

ABSTRACT

A communication system allows for clock synchronization between a transmitter and a receiver when switching from transmission of an analog signal to transmission of a digital signal. The system uses clock synchronization during transmission of the digital signal, but the clock synchronization may be lost when switching to transmission of an analog signal. A digital clock synchronization is embedded in the analog signal so that the clock synchronization between the transmitter and the receiver may be reestablished upon switching to a digital signal without any delay in transmission of the digital signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/929,271, filed Apr. 21, 2020, which is hereby incorporated byreference. U.S. patent application Ser. No. 15/929,271, filed Apr. 21,2020, is a continuation of International Patent Application NumberPCT/US2018/057022, filed Oct. 23, 2018, which are hereby incorporated byreference. International Patent Application Number PCT/US2018/057022,filed Oct. 23, 2018, claims the benefit of U.S. Patent Application No.62/576,253, filed Oct. 24, 2017, which are hereby incorporated byreference.

BACKGROUND

The Federal Communications Commission (FCC) requires broadcast stationsto provide identification at the beginning and end of each period ofoperation as well as hourly, as close to the beginning of the hour aspossible. Typically the station identifier is broadcast in an AM or FMformat, so that a digital broadcast system has to switch to an AmplitudeModulation (AM) or Frequency Modulation (FM) signal to provideidentification and then switch back to digital to resume digitaltransmission. When switching between digital transmission and an AM orFM transmission, the clock synchronization between the transmitter andthe receiver may be interrupted. Clock synchronization may bereestablished after switching back to digital transmission; however,this process may take time and cause a delay during synchronization.Thus, there is a need for improvement in this field.

SUMMARY

A communication system uses may be used to provide a low latencycommunication pathway for data between remotely located transmitter andreceiver stations, such as located in North America and Europe.Generally speaking, the overall system operates in two modes, a datatransmission mode and a broadcast mode. In the data transmission mode,data is transmitted in a digital form between the transmitter andreceiver. On off hours when data transmission is not needed, such aswhen stock exchanges are closed, the transmitter broadcasts music, news,entertainment, and/or other audio information to the general public,such as via an AM or FM signal, or even a phase modulation (PM) signal.In one form, the digital data signal and the AM signal are transmittedat different frequencies, but in other variations, they can share thesame general frequency band.

When in a data transmission mode, the system attempts to communicate thedata as fast as physically possible, that is, with the lowest practicallatency. Normally, but not always, the data is transmitted using adigital signal which requires clock synchronization between thetransmitter and receiver. The clocks at the transmitter and receivertypically set their clocks at least in part based on time data receivedfrom a Global Positioning System (GPS). During transmission, the radiodata signal is bounced off the ionosphere. Due to changes in ionosphericconditions and other environmental changes, the path length of thedigital signal between the transmitter and the receiver changes which inturn leads to signal distortions such as Doppler shifting. So thereceiver is able to remove or adjust to these distortions, thetransmitted digital signal includes a clock signal that helpssynchronize the transmitter and receiver.

As part of the broadcast license, the FCC requires broadcasters tointerrupt their normally scheduled programming in order to provide astation identifier (ID) on an hourly basis (or at other intervals). Thestation ID may be transmitted using the AM broadcast signal. While thisis usually not a problem during the broadcast mode as latency is not aconcern, during the data transmission mode, the receiver has to switchfrom the digital signal to the AM signal. During this switch to the AMsignal, the clock synchronization signal is lost. Upon switching back tothe digital data transmission signal, there is a delay caused byreestablishing the clock synchronization between the transmitter andreceiver. This delay can be problematic because the informationtransmitted via the digital data signal is extremely time sensitive.

To address this issue, a digital low bit rate clock synchronizationsignal is embedded in the AM station ID signal so that clocksynchronization between the transmitter and receiver is maintained. Thisin turn eliminates the delay caused by the reestablishment of the clocksynchronization when switching from the AM station ID signal back to thedigital data transmission signal. In one form, the digital low bit rateclock synchronization signal is modulated using phase modulation, but itcan be modulated in other ways. Normally, the digital low bit rate clocksynchronization signal is transmitted during the entire station ID break(i.e., when the AM station ID signal is broadcasted), but in othervariations, the digital low bit rate clock synchronization signal can betransmitted just before reestablishment of the digital data transmissionsignal so long as clock synchronization is established on or before datatransmission occurs without any delay.

This technique can be also used to maintain clock synchronization whenchanging from the broadcast mode to the data transmission mode. In aparticular example, the digital low bit rate clock synchronizationsignal can be transmitted during a music broadcast before trading on thestock exchanges start so that the transmitter and receiver areimmediately synchronized when switched to the data transmission mode. Inone variation, the digital low bit rate clock synchronization signal istransmitted during the entire broadcast, and in another variation, it istransmitted just before switching to the data transmission mode, like inthe earlier example.

In a more general sense, this technique can be used to maintain orreestablish clock synchronization without delay when switching from ananalog signal, such as audio and/or older analog television (NTSC orPAL) broadcast signal, to a digital signal, such as for datatransmission or other forms of communication that need low latency.

While the system will be described with reference to executing financialtrading strategies, this system and technique can be used in othersituations or industries where time and bandwidth are of concern. Forexample, this system can be used to perform remote surgery or medicaldiagnostics, scientific instruments or studies (e.g., for astronomy orphysics), controlling dispersed global computer networks, and/ormilitary applications. This system and technique can for example beadapted for incorporation into earthquake/tsunami early warning systems.

Further forms, objects, features, aspects, benefits, advantages, andembodiments of the present invention will become apparent from adetailed description and drawings provided herewith.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system for transmitting data overseparate communication links, one of which uses skywave propagation.

FIG. 2 is a schematic diagram further illustrating the skywavepropagation of FIG. 1

FIG. 3 is a schematic diagram illustrating the use of ground-basedrepeaters in the skywave propagation of FIG. 1 .

FIG. 4 is a schematic diagram illustrating the use of airborne repeatersin the skywave propagation of FIG. 1 .

FIG. 5 is a schematic diagram illustrating additional layers of theatmosphere including the ionized layer shown in FIG. 1 .

FIG. 6 is a schematic diagram illustrating various ionized layers of theatmosphere shown in FIG. 5 .

FIG. 7 is a schematic diagram illustrating additional details of skywavepropagation generally illustrated in FIGS. 1-6 .

FIG. 8 is a schematic diagram illustrating additional detail for thecommunication nodes of FIG. 1 .

FIG. 9 is a schematic diagram illustrating additional detail for the RFcommunication interface in FIG. 8 .

FIG. 10 is a schematic diagram of the transmission of a digital signalbetween a transmitter and a receiver.

FIG. 11 is a schematic diagram of the transmission of an analog signaland a clock synchronization signal between the transmitter and thereceiver of FIG. 10 .

FIG. 12 is a schematic diagram of switching from the transmission of ananalog signal to a digital signal between the transmitter and thereceiver of FIG. 10 .

FIG. 13 is flowchart for a method for reducing latency due to clocksynchronization.

DESCRIPTION OF THE SELECTED EMBODIMENTS

For the purpose of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the described embodiments, and any further applications of theprinciples of the invention as described herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates. One embodiment of the invention is shown in great detail,although it will be apparent to those skilled in the relevant art thatsome features that are not relevant to the present invention may not beshown for the sake of clarity.

FIG. 1 illustrates one example of a communication system 100 configuredto transfer data via a low latency, low bandwidth communication link104, and separate data via a high latency, high bandwidth communicationlink 108. The communication links 104 and 108 provide separateconnections between a first communication node 112 and a secondcommunication node 116. The low latency connection 104 may be configuredto transmit data using electromagnetic waves 124 passing through freespace via skywave propagation. The electromagnetic waves 124 may begenerated by a transmitter in the first communication node 112, passedalong a transmission line 136 to an antenna 128. The electromagneticwaves 124 may be radiated by the antenna 128 encountering an ionizedportion of the atmosphere 120. This radiated electromagnetic energy maythen be refracted by the ionized portion of the atmosphere 120 causingthe waves 124 to redirect toward earth. The waves 124 may be received bya receiving antenna 132 coupled to the second communications node 116 bythe transmission line 140. As illustrated in FIG. 1 , a transmittingcommunication node may use skywave propagation to transmitelectromagnetic energy long distances across the earth surface withoutthe need of one or more transmission lines to carry the electromagneticenergy.

Data may also be transmitted between the communications nodes 112 and116 using a high latency communication link 108. As illustrated in FIG.1 , the high latency communication link 108 may be implemented using atransmission line 144 passing through the earth, which may includepassing under or through an ocean or other body of water. As shown inFIG. 1 , the high latency communication link 108 may include repeaters152. FIG. 1 illustrates four repeaters 152 along the transmission line144 although any suitable number of repeaters 152 may be used. Thetransmission line 144 may also have no repeaters at all. Although FIG. 1illustrates the communication link 104 transmitting information from thefirst communication node 112 to the second communication node 116, thedata transmitted may pass along the communication links 104, 108 in bothdirections.

A client 160 may have a connection 164 to the first communication node112. The client 160 may send instructions over the connection 164 to thefirst communication node 112. At the first communication node 112, theinstructions are prepared to be sent to the second communication node116, either by the low latency link 104 or the high latency link 108, orboth. The second communication node 116 may be connected to aninstruction processor 168 by a connection 172. The client 160 may be anybusiness, group, individual, or entity that desires to send directionsover a distance. The instruction processor 168 may be any business,group, individual, or entity that is meant to receive or act upon thoseinstructions. In some embodiments, the connections 164 and 172 may beunnecessary as the client may send the data to be transmitted directlyfrom the communication node 112 or the communication node 116 may beconnected directly to the instruction processor 168. The system 100 maybe used for any kind of low-latency data transmission that is desired.As one example, the client 160 may be a doctor or surgeon workingremotely while the instruction processor 168 may be a robotic instrumentfor working on a patient.

In some embodiments, the client 160 may be a financial instrument traderand the instruction processor 168 may be a stock exchange. The tradermay wish to provide instructions to the stock exchange to buy or sellcertain securities or bonds at specific times. The trader may transmitthe instructions to the first communication node 112 which sends theinstructions to second communication node using the antennae 128, 132 orby the transmission line 144. The stock exchange can then process theactions desired by the trader upon receipt of the instructions.

The system 100 may be useful for high-frequency trading, where tradingstrategies are carried out on computers to execute trades in fractionsof a second. In high-frequency trading, a delay of mere milliseconds maycost a trader millions of dollars; therefore, the speed of transmissionof trading instructions is as important as the accuracy of the datatransmitted. In some embodiments, the trader may transmit preset tradinginstructions or conditions for executing a trade to the communicationnode 116, which is located within close proximity to a stock exchange,using the high latency, high bandwidth communication link 108 at a timebefore the trader wishes to execute a trade. These instructions orconditions may require the transmission of a large amount of data, andmay be delivered more accurately using the higher bandwidthcommunication link 108. Also, if the instructions or conditions are sentat a time prior to when a trade is wished to be executed, the higherlatency of the communication link 108 can be tolerated.

The eventual execution of the instructions may be accomplished by thetrader transmitting triggering data to the system on which theinstructions are stored. Upon receipt of the triggering data, thetrading instructions are sent to the stock exchange and a trade isexecuted. The triggering data that is transmitted is generally a muchsmaller amount of data than the instructions; therefore, the triggeringdata may be sent over the low latency, low bandwidth communication link104. When the triggering data is received at communication node 116, theinstructions for a specific trade are sent to the stock exchange.Sending the triggering data over the low latency communication link 104rather than the high latency communication link 108 allows the desiredtrade to be executed as quickly as possible, giving the trader a timeadvantage over other parties trading the same financial instruments.

The configuration shown in FIG. 1 is further illustrated in FIG. 2 wherethe first communication node 112 and the second communication node 116are geographically remote from one another separated by a substantialportion of the surface of the earth (156). This portion of the earth'ssurface may include one or more continents, oceans, mountain ranges, orother geographic areas. For example, the distance spanned in FIGS. 1-7may cover a single continent, multiple continents, an ocean, and thelike. In one example, the first communication node 112 is in Chicago,Ill. in the United States of America, and the second communication node116 is in London, England, in the United Kingdom. In another example,the first communication node 112 is in New York City, N.Y., and secondcommunication node 116 is in Los Angeles, Calif., both cities being inNorth America. Any suitable combination of distance, communicationnodes, and communications links is envisioned that can providesatisfactory latency and bandwidth.

FIG. 2 illustrates that skywave propagation allows electromagneticenergy to traverse long distances. Using skywave propagation, the lowlatency communication link 104 transmits the electromagnetic waves 124into a portion of the atmosphere 120 that is sufficiently ionized torefract the electromagnetic waves 124 toward the earth. The waves maythen be reflected by the surface of the earth and returned to theionized portion of the upper atmosphere 120 where they may be refractedtoward earth again. Thus electromagnetic energy may “skip” repeatedlyallowing the low latency, low bandwidth signals 124 to cover distancessubstantially greater than those which may be covered by non-skywavepropagation.

Another example of the system illustrated in FIG. 1 appears in FIG. 3where the skywave propagation discussed with respect to FIGS. 1 and 2may be enhanced using repeaters 302 and 306. In this example, the firstrepeater 302 may receive the low latency communication signals emanatingfrom the antenna 128. The signals may be refracted by the ionized region120 and returned to earth where they may be received by the repeater 302and retransmitted via skywave propagation. The refracted signal may bereceived by the repeater 306 and retransmitted using skywave propagationto the second communications node 116 via the antenna 132. Although tworepeating stations are illustrated in FIG. 3 , any suitable number,configuration, or positioning of the ground repeating stations 302 isconsidered. Increasing the number of repeaters 302, 306 may provide forthe opportunity to transmit low latency signals over greater distancesin a wider array of atmospheric missions, however, the physicallimitations of the repeater circuitry that receives and retransmits thesignal may add additional latency to low latency communication link 104.

FIG. 4 illustrates another example of the system illustrated in FIG. 1where one or more repeaters along the first communications link areairborne, such as in an aircraft, dirigible, balloon, or other device410 configured to maintain the repeater aloft in the atmosphere. In thisexample, signals transmitted from the first communications node 112 viathe antenna 128 may be received by an airborne repeater 414 either asline of sight communication 402, or by skywave propagation as describedherein elsewhere. The signals may be received by the airborne repeater414 and retransmitted as line of sight communication 406, or by skywavepropagation to the second communications node 116 along the low latencylink 104.

Additional details regarding skywave propagation are illustrated inFIGS. 5-7 . The relation to the system disclosed and various layers ofthe upper atmosphere is illustrated in FIG. 5 . For purposes of radiotransmission, the layers of the upper atmosphere may be divided as showninto successively higher layers such as the troposphere 504, thestratosphere 508, and the ionosphere 512.

The ionosphere is named as such because it includes a high concentrationof ionized particles. The density of these particles in the ionospherefurthest from earth is very low and becomes progressively higher in theareas of the ionosphere closer to earth. The upper region of theionosphere is energized by powerful electromagnetic radiation from thesun which includes high-energy ultraviolet radiation. This solarradiation causes ionization of the air into free electrons, positiveions, and negative ions. Even though the density of the air molecules inthe upper ionosphere is low, the radiation particles from space are ofsuch high energy that they cause extensive ionization of the relativelyfew air molecules that are present. The ionization extends down throughthe ionosphere with diminishing intensity as air becomes denser with thehighest degree of ionization thus occurring at the upper extremities ofthe ionosphere, while the lowest degree occurs in the lower portion ofthe ionosphere.

These differences in ionization between the upper and lower extremitiesof the ionosphere 512 are further illustrated in FIG. 6 . The ionosphereis illustrated in FIG. 6 with three layers designated, respectively,from lowest level to highest level as D layer 608, E layer 612, and Flayer 604. The F layer 604 may be further divided into two layersdesignated F1 (the higher layer) at 616 and F2 (the lower layer) at 620.The presence or absence of layers 616 and 620 in the ionosphere andtheir height above the earth vary with the position of the sun. At highnoon, radiation from the sun 624 passing into the ionosphere isgreatest, tapering off at sunset and at a minimum at night. When theradiation is removed, many of the ions recombine causing the D layer 608and the E layer 612 to disappear, and further causing the F1 and F2layers 616, 620 to recombine into a single F layer 604 during the night.Since the position of the sun varies with respect to a given point onearth, the exact characteristics of the layers 608, 612, 616, and 620 ofthe ionosphere 512 can be extremely difficult to predict but may bedetermined by experimentation.

The ability for a radio wave to reach a remote location using skywavepropagation depends on various factors such as ion density in the layers608-620 (when they are present), the frequency of the transmittedelectromagnetic energy, and the angle of transmission. For example, ifthe frequency of a radio wave is gradually increased, a point will bereached where the wave cannot be refracted by the D layer 608 which isthe least ionized layer of the ionosphere 512. The wave may continuethrough the D layer 608 and into the E layer 612 where its frequency maystill be too great to refract the singles passing through this layer aswell. The waves 124 may continue to the F2 layer 620 and possibly intothe F1 layer 616 as well before they are bent toward earth. In somecases, the frequency may be above a critical frequency making itimpossible for any refraction to occur causing the electromagneticenergy to be radiated out of the earth's atmosphere (708).

Thus, above a certain frequency, electromagnetic energy transmittedvertically continues into space and is not refracted by the ionosphere512. However, some waves below the critical frequency may be refractedif the angle of propagation 704 is lowered from the vertical. Loweringthe angle of propagation 704 also allows the electromagnetic waves 124transmitted by the antenna 128 to be refracted toward Earth's surfacewithin a skip zone 720 making it possible to traverse a skip distance724 and reach a remote antenna 132. Thus the opportunity for successfulskywave propagation over a certain skip distance 724 is furtherdependent on the angle of transmission as well as the frequency, andtherefore the maximum usable frequency varies with the condition of theionosphere, desired skip distance 724, and the propagation angle 704.FIG. 7 also illustrates that non-skywave propagation such as groundwavesignals and/or line of sight signals 716 are unlikely to traverse theskip distance 724.

FIG. 8 illustrates one example of additional aspects of a communicationnode 800 which is like the communication nodes 112 and 116. Thecommunication node 800 can include a processor 804 for controllingvarious aspects of communication node 800. The processor may be coupledto a memory 816 useful for storing rules, command data 820, orhistorical transmission data 822. Devices for accepting user input andproviding output (I/O) to a user 824 may also be included. These devicesmay include a keyboard or keypad, a mouse, a display such as a flatpanel monitor and the like, a printer, plotter, or 3D printer, a camera,or a microphone. Any suitable devices for user I/O may be included. Node800 may also include a network interface 832 responsive to the processor804 and coupled to a communication network 836. A security module 828may be included as well and may be used to reduce or eliminate theopportunity for third-parties to intercept, jam, or change data as itpasses between the communications nodes 800. In one example, thecommunication node 800 is implemented as a computer executing softwareto control the interaction of the various aspects of the node 800.

The network interface 836 may be configured to send and receive datasuch as command data 820, or triggering data which may be passed from atriggering system 840. The communication network 836 may be coupled to anetwork such as the internet and configured to send and receive datawithout the use of skywave propagation. For example, the communicationnetwork 836 may transmit and receive data over optical fibers or othertransmission lines running along the earth similar to the transmissionlines 144 illustrated in previous figures.

The node 800 may include a second network interface 808 responsive tothe processor 804 and coupled to a radio-frequency communicationinterface 812. This second network interface 808 may be used to transferdata such as command data 820 or triggering data passed from thetriggering system 840. The network interface 808 may be coupled to anantenna like antenna 128 which may include multiple antennas or antennaelements. The radio-frequency communication interface 808 may beconfigured to send and receive data such as triggering data usingelectromagnetic waves transmitted and/or received via the antenna 128.As discussed above, the antenna 128 may be configured to send andreceive the electromagnetic waves via skywave propagation.

The communication node 800 may include additional aspects illustrated inFIG. 9 . The radio-frequency communication interface 812 may include atransmitter 904 configured to transmit electromagnetic energy using theantenna 128. A receiver 908 may optionally be included as well andconfigured to receive electromagnetic waves from the antenna 128. Thetransmitter 904 and the receiver 908 may also be coupled to a modem 912configured to modulate signals received by the interface 812 to encodeinformation or data from a digital stream for transmission bytransmitter 904. The modem 912 may also be configured to demodulatesignals received by the receiver 908 from the antenna 128 to decode thetransmitted signal into a digital data stream usable by the processor804 or that may be stored in the memory 816.

In some embodiments, data transmission for purposes such as financialtrading may only be needed for a limited period of time. Thecommunication system 100 may operate in two different modes depending onwhether financial data transmission is needed. If data transmission isnecessary, the transmitter may operate in a data transmission mode. Indata transmission mode, the communication system 100 transmits a digitaldata transmission signal from the first communication node 112 to thesecond communication node 116. When data transmission is not required,the system may operate in a broadcast mode. During broadcast mode, thetransmitter may be used to broadcast music, sports, news, or other audioinformation to the general public, such as via an AM, FM, and/or PMsignal. While the communication system 100 is operated in broadcastmode, it may broadcast in a digital form. For the purposes forexplanation, the broadcast mode for the communication system will bedescribed with reference to an AM signal, but it should be appreciatedthat AM, FM, and/or PM (or the like) can be used during the broadcastmode.

As a specific example, the communication system 100 may be used fortransmitting instructions for financial transactions such as buying andselling stocks or bonds. This information would only be necessary duringthe hours that the desired stock exchanges are open. During off hours,when the stock exchanges are closed, the communication system 100 may beoperated as a typical radio station, for example via an AM signal,providing content that may be heard by the general public.

Typically, when data is transmitted using a digital signal, clocksynchronization is needed between the transmitter and the receiver.Because the path length of the transmitted data signal may changedepending on the ionospheric conditions and other environmentalconditions, resulting in signal distortion such as Doppler shifting. Inorder to adjust for or remove these distortions, the transmitted datasignal may include a clock signal that provides information to thereceiver regarding the transmission time of the data signal and possibledistortion. In some embodiments, the clock for the transmitter and theclock for the receiver may be set at least in part based on time datareceived from a Global Positioning System (GPS). However, other sourcesof time data other than GPS may be included in the transmitted datasignal.

One source that may interrupt the clock synchronization between atransmitter and a receiver is the need for station identification (ID).The FCC requires broadcast stations to provide station identificationupon sign on, sign off, and near the top of every hour. The station isrequired to provide its call letters, city of license, and may includeadditional information such as transmission frequencies and ownership.Typically, station identification is transmitted using an analog signal,such as an AM broadcast signal. Therefore, in order to provideidentification, the receiver switches from a digital signal to an AMsignal. During the switch to the AM signal, the clock synchronizationsignal provided on the digital signal is lost. Once the receiver isswitched back to the digital signal, there may be a delay as clocksynchronization is reestablished between the transmitter and thereceiver. This can cause processing delays within the transmitter and/orthe receiver.

While the communication system 100 is in broadcast mode, the need forstation identification does not present an issue, as the signal isalready being broadcast as an AM signal, and any delay is not a concern.However, when the communication system 100 is operated in the digitaldata transmission mode, the switch from digital transmission to AMtransmission to provide station identification may cause the transmitterand receiver to lose clock synchronization and there may be a delay assynchronization is reestablished. The delay caused by synchronizationmay be problematic depending on the nature of the digital datatransmitted by the communication system 100 while in data transmissionmode. For example, when the communication system 100 is used fortransmission of financial trading data, the data may be extremely timesensitive and any delay may cost a client millions of dollars.

A system 1000 for reducing latency due to clock synchronization is shownin FIGS. 10-12 . In FIG. 10 , a transmitter 1004 includes an analogtransmitter 1008, a digital transmitter 1012, and a transmitter clock1016. A receiver 1024 includes an analog receiver 1028, a digitalreceiver 1032, and a receiver clock 1036. When transmitter 1004 issending digital data to receiver 1024, the digital transmitter 1012transmits a digital signal 1042 to the digital receiver 1032. It iscommon for data transmitted using a digital signal to require clocksynchronization between the transmitter clock 1016 and the receiverclock 1036 by using a signal transmitted with the digital data or byusing other signals such as time data received from a GPS.

At times, it may be necessary to switch from digital transmission toanalog transmission, such as when station identification is required. Asshown in FIG. 11 , when analog transmission is desired, the digitaltransmission is stopped and an analog signal 1104 is sent from theanalog transmitter 1008 to the analog receiver 1028. In some typicalcommunication systems, clock synchronization between the transmitterclock 1016 and the receiver clock 1036 may be lost during analogtransmission. However, in system 1000, a clock synchronization signal1108 may be transmitted with the analog signal 1104 to maintainsynchronization between the transmitter clock 1016 and the receiverclock 1036. In one example, the clock synchronization signal 1108 is adigital low-bit rate signal, but clock synchronization signal 1108 inother examples can alternatively or additionally include high-bit ratesignals and/or analog signals.

Once transmission of the analog signal 1104 has been completed, thesystem switches back to transmission of the digital signal 1042, asshown in FIG. 12 . The clock synchronization signal 1108 maintainssynchronization between the transmitter clock 1016 and the receiver 1036during the switch between the analog signal 1104 and the digital signal1042. Therefore, there is no delay to reestablish clock synchronizationduring the switch between analog and digital signals.

The flowchart 1300 in FIG. 13 describes the method for reducing delaysdue to loss of clock synchronization. The communication system 100transmits 1305 data as a digital data transmission from a transmitter toa receiver, such as from first communication node 112 to secondcommunication node 116. At the specified times during the day, thetransmission from first communication node 112 is switched 1310 from adigital data transmission to an AM station ID signal to provide stationidentification as required by law. A digital clock synchronizationsignal is embedded 1315 within the AM station ID signal and broadcast asthe station identification is broadcast. After station identification iscomplete, the communication system 100 switches 1320 from transmissionof the AM station ID signal to resume transmission of the digital datatransmission. As the transmission is switched from the AM station IDsignal to the digital data stream, clock synchronization is maintained1025 due to the digital clock synchronization signal that was embeddedin the AM station ID signal.

In some embodiments, the clock synchronization signal embedded in the AMstation ID signal may be modulated using PM. However, the clocksynchronization signal may be modulated in other ways such as via FMand/or AM. Also, in some embodiments, the clock synchronization signalmay be embedded in the station ID signal so that the clocksynchronization signal is broadcast during the entire stationidentification break. However, in other embodiments, the clocksynchronization signal may be transmitted only during the portion of thestation identification directly before reestablishment of the digitaldata transmission signal, so that clock synchronization is achievedbefore the transmission is switched from AM broadcast to digitaltransmission.

Clock synchronization may also be used as the communication system 100switches from broadcast mode to data transmission mode. As thecommunication system 100 broadcasts to the general public, a digitalclock synchronization signal may be transmitted along with the music,news, sports, or other form of audio information is broadcast to thepublic. The synchronization signal may be transmitted only in the timeleading up to the switch to data transmission mode, or the clocksynchronization signal may be transmitted for the entire time that thecommunication system 100 is operating in broadcast mode. Once thecommunication system 100 is switched from broadcast mode to datatransmission mode, the clock synchronization signal allows the firstcommunication node 112 and second communication node 116 to beimmediately synchronized so that there is no or minimum delay caused bysynchronization.

The embedded synchronization technique described above for maintainingclock synchronization or reestablishing clock synchronization to avoid adelay may also be used for other types of broadcasts that require lowlatency. For example, embedded clock synchronization may be used whenswitching from an analog signal, such as audio or an analog televisionsignal to a digital signal.

GLOSSARY OF DEFINITIONS AND ALTERNATIVES

The language used in the claims and specification is to only have itsplain and ordinary meaning, except as explicitly defined below. Thewords in these definitions are to only have their plain and ordinarymeaning. Such plain and ordinary meaning is inclusive of all consistentdictionary definitions from the most recently published Webster's andRandom House dictionaries. As used in the specification and claims, thefollowing definitions apply to the following terms or common variationsthereof (e.g., singular/plural forms, past/present tenses, etc.).

“Analog signal” generally refers to a continuous signal of atime-varying variable. An analog signal may be modulated using amplitudemodulation (AM), frequency modulation (FM), or other forms ofmodulation, such as phase modulation.

“Antenna” or “Antenna system” generally refers to an electrical device,or series of devices, in any suitable configuration, that convertselectric power into electromagnetic radiation. Such radiation may beeither vertically, horizontally, or circularly polarized at anyfrequency along the electromagnetic spectrum. Antennas transmitting withcircular polarity may have either right-handed or left-handedpolarization.

In the case of radio waves, an antenna may transmit at frequenciesranging along electromagnetic spectrum from extremely low frequency(ELF) to extremely high frequency (EHF). An antenna or antenna systemdesigned to transmit radio waves may comprise an arrangement of metallicconductors (elements), electrically connected (often through atransmission line) to a receiver or transmitter. An oscillating currentof electrons forced through the antenna by a transmitter can create anoscillating magnetic field around the antenna elements, while the chargeof the electrons also creates an oscillating electric field along theelements. These time-varying fields radiate away from the antenna intospace as a moving transverse electromagnetic field wave. Conversely,during reception, the oscillating electric and magnetic fields of anincoming electromagnetic wave exert force on the electrons in theantenna elements, causing them to move back and forth, creatingoscillating currents in the antenna. These currents can then be detectedby receivers and processed to retrieve digital or analog signals ordata.

Antennas can be designed to transmit and receive radio wavessubstantially equally in all horizontal directions (omnidirectionalantennas), or preferentially in a particular direction (directional orhigh gain antennas). In the latter case, an antenna may also includeadditional elements or surfaces which may or may not have any physicalelectrical connection to the transmitter or receiver. For example,parasitic elements, parabolic reflectors or horns, and other suchnon-energized elements serve to direct the radio waves into a beam orother desired radiation pattern. Thus antennas may be configured toexhibit increased or decreased directionality or “gain” by the placementof these various surfaces or elements. High gain antennas can beconfigured to direct a substantially large portion of the radiatedelectromagnetic energy in a given direction that may be verticalhorizontal or any combination thereof.

Antennas may also be configured to radiate electromagnetic energy withina specific range of vertical angles (i.e. “takeoff angles) relative tothe earth in order to focus electromagnetic energy toward an upper layerof the atmosphere such as the ionosphere. By directing electromagneticenergy toward the upper atmosphere at a specific angle, specific skipdistances may be achieved at particular times of day by transmittingelectromagnetic energy at particular frequencies.

Other examples of antennas include emitters and sensors that convertelectrical energy into pulses of electromagnetic energy in the visibleor invisible light portion of the electromagnetic spectrum. Examplesinclude light emitting diodes, lasers, and the like that are configuredto generate electromagnetic energy at frequencies ranging along theelectromagnetic spectrum from far infrared to extreme ultraviolet.

“Clock synchronization” generally refers to the coordination of timebetween otherwise independent clocks, computers, processors.transceivers, and/or other types of devices. For example, clocksynchronization is used to compensate for clock drift between differentclocks.

“Clock synchronization signal” generally refers to a signal thatprovides time data for synchronizing a transmitter and a receiver. Theclock synchronization signal may be received from a Global PositioningSystem and/or may originate from some other public and/or privatesource.

“Command” or “Command Data” generally refers to one or more directives,instructions, algorithms, or rules controlling a machine to take one ormore actions, alone or in combination. A command may be stored,transferred, transmitted, or otherwise processed in any suitable manner.For example, a command may be stored in a memory or transmitted over acommunication network as electromagnetic radiation at any suitablefrequency passing through any suitable medium.

“Computer” generally refers to any computing device configured tocompute a result from any number of input values or variables. Acomputer may include a processor for performing calculations to processinput or output. A computer may include a memory for storing values tobe processed by the processor, or for storing the results of previousprocessing.

A computer may also be configured to accept input and output from a widearray of input and output devices for receiving or sending values. Suchdevices include other computers, keyboards, mice, visual displays,printers, industrial equipment, and systems or machinery of all typesand sizes. For example, a computer can control a network interface toperform various network communications upon request. The networkinterface may be part of the computer, or characterized as separate andremote from the computer.

A computer may be a single, physical, computing device such as a desktopcomputer, a laptop computer, or may be composed of multiple devices ofthe same type such as a group of servers operating as one device in anetworked cluster, or a heterogeneous combination of different computingdevices operating as one computer and linked together by a communicationnetwork. The communication network connected to the computer may also beconnected to a wider network such as the internet. Thus computer mayinclude one or more physical processors or other computing devices orcircuitry, and may also include any suitable type of memory.

A computer may also be a virtual computing platform having an unknown orfluctuating number of physical processors and memories or memorydevices. A computer may thus be physically located in one geographicallocation or physically spread across several widely scattered locationswith multiple processors linked together by a communication network tooperate as a single computer.

The concept of “computer” and “processor” within a computer or computingdevice also encompasses any such processor or computing device servingto make calculations or comparisons as part of disclosed system.Processing operations related to threshold comparisons, rulescomparisons, calculations, and the like occurring in a computer mayoccur, for example, on separate servers, the same server with separateprocessors, or on a virtual computing environment having an unknownnumber of physical processors as described above.

A computer may be optionally coupled to one or more visual displaysand/or may include an integrated visual display. Likewise, displays maybe of the same type, or a heterogeneous combination of different visualdevices. A computer may also include one or more operator input devicessuch as a keyboard, mouse, touch screen, laser or infrared pointingdevice, or gyroscopic pointing device to name just a few representativeexamples. Also, besides a display, one or more other output devices maybe included such as a printer, plotter, industrial manufacturingmachine, 3D printer, and the like. As such, various display, input andoutput device arrangements are possible.

Multiple computers or computing devices may be configured to communicatewith one another or with other devices over wired or wirelesscommunication links to form a communication network. Networkcommunications may pass through various computers operating as networkappliances such as switches, routers, firewalls or other network devicesor interfaces before passing over other larger computer networks such asthe internet. Communications can also be passed over the communicationnetwork as wireless data transmissions carried over electromagneticwaves through transmission lines or free space. Such communicationsinclude using WiFi or other Wireless Local Area Network (WLAN) or acellular transmitter/receiver to transfer data. Such signals conform toany of a number of wireless or mobile telecommunications technologystandards such as 802.11a/b/g/n, 3G, 4G, and the like.

“Communication Link” generally refers to a connection between two ormore communicating entities and may or may not include a communicationschannel between the communicating entities. The communication betweenthe communicating entities may occur by any suitable means. For examplethe connection may be implemented as an actual physical link, anelectrical link, an electromagnetic link, a logical link, or any othersuitable linkage facilitating communication.

In the case of an actual physical link, communication may occur bymultiple components in the communication link figured to respond to oneanother by physical movement of one element in relation to another. Inthe case of an electrical link, the communication link may be composedof multiple electrical conductors electrically connected to form thecommunication link.

In the case of an electromagnetic link, elements the connection may beimplemented by sending or receiving electromagnetic energy at anysuitable frequency, thus allowing communications to pass aselectromagnetic waves. These electromagnetic waves may or may not passthrough a physical medium such as an optical fiber, or through freespace, or any combination thereof. Electromagnetic waves may be passedat any suitable frequency including any frequency in the electromagneticspectrum.

In the case of a logical link, the communication link may be aconceptual linkage between the sender and recipient such as atransmission station in the receiving station. Logical link may includeany combination of physical, electrical, electromagnetic, or other typesof communication links.

“Communication node” generally refers to a physical or logicalconnection point, redistribution point or endpoint along a communicationlink. A physical network node is generally referred to as an activeelectronic device attached or coupled to a communication link, eitherphysically, logically, or electromagnetically. A physical node iscapable of sending, receiving, or forwarding information over acommunication link. A communication node may or may not include acomputer, processor, transmitter, receiver, repeater, and/ortransmission lines, or any combination thereof.

“Critical angle” generally refers to the highest angle with respect to avertical line extending to the center of the Earth at which anelectromagnetic wave at a specific frequency can be returned to theEarth using sky-wave propagation.

“Critical Frequency” generally refers to the highest frequency that willbe returned to the Earth when transmitted vertically under givenionospheric conditions using sky-wave propagation.

“Data Bandwidth” generally refers to the maximum throughput of a logicalor physical communication path in a communication system. Data bandwidthis a transfer rate that can be expressed in units of data transferredper second. In a digital communications network, the units of datatransferred are bits and the maximum throughput of a digitalcommunications network is therefore generally expressed in “bits persecond” or “bit/s.” By extension, the terms “kilobit/s” or “Kbit/s”,“Megabit/s” or “Mbit/s”, and “Gigabit/s” or “Gbit/s” can also be used toexpress the data bandwidth of a given digital communications network.Data networks may be rated according to their data bandwidth performancecharacteristics according to specific metrics such as “peak bit rate”,“mean bit rate”, “maximum sustained bit rate”, “information rate”, or“physical layer useful bit rate.” For example, bandwidth tests measurethe maximum throughput of a computer network. The reason for this usageis that according to Hartley's Law, the maximum data rate of a physicalcommunication link is proportional to its frequency bandwidth in hertz.

Data bandwidth may also be characterized according to the maximumtransfer rate for a particular communications network. For example:

“Low Data Bandwidth” generally refers to a communications network with amaximum data transfer rate that is less than or about equal to 1,000,000units of data per second. For example, in a digital communicationsnetwork, the unit of data is a bit. Therefore low data bandwidth digitalcommunications networks are networks with a maximum transfer rate thatis less than or about equal to 1,000,000 bits per second (1 Mbits/s).

“High Data Bandwidth” generally refers to a communications network witha maximum data transfer rate that is greater than about 1,000,000 unitsof data per second. For example, a digital communications network with ahigh data bandwidth is a digital communications network with a maximumtransfer rate that is greater than about 1,000,000 bits per second (1Mbits/s).

“Demodulator” generally refers to any device that decodes a modulatedwaveform or any another type carrier wave that has been transmitted. Ademodulator may be used to allow decoding of either analog or digitalsignals.

“Digital signal” generally refers to a signal containing data that istransmitted using a sequence of discrete values.

“Electromagnet Radiation” generally refers to energy radiated byelectromagnetic waves. Electromagnetic radiation is produced from othertypes of energy, and is converted to other types when it is destroyed.Electromagnetic radiation carries this energy as it travels moving awayfrom its source at the speed of light (in a vacuum). Electromagneticradiation also carries both momentum and angular momentum. Theseproperties may all be imparted to matter with which the electromagneticradiation interacts as it moves outwardly away from its source.

Electromagnetic radiation changes speed as it passes from one medium toanother. When transitioning from one media to the next, the physicalproperties of the new medium can cause some or all of the radiatedenergy to be reflected while the remaining energy passes into the newmedium. This occurs at every junction between media that electromagneticradiation encounters as it travels.

The photon is the quantum of the electromagnetic interaction, and is thebasic constituent of all forms of electromagnetic radiation. The quantumnature of light becomes more apparent at high frequencies aselectromagnetic radiation behaves more like particles and less likewaves as its frequency increases.

“Electromagnetic Spectrum” generally refers to the range of all possiblefrequencies of electromagnetic radiation. The electromagnetic spectrumis generally categorized as follows, in order of increasing frequencyand energy and decreasing wavelength:

“Extremely low frequency” (ELF) generally designates a band offrequencies from about 3 to about 30 Hz with wavelengths from about100,000 to 10,000 km long.

“Super low frequency” (SLF) generally designates a band of frequenciesgenerally ranging between about 30 Hz to about 300 Hz with wavelengthsof about 10,000 to about 1000 km long.

“Voice frequency” or “voice band” generally designates electromagneticenergy that is audible to the human ear. Adult males generally speak inthe range between about 85 and about 180 Hz while adult femalesgenerally converse in the range from about 165 to about 255 Hz.

“Very low frequency” (VLF) generally designates the band of frequenciesfrom about 3 kHz to about 30 kHz with corresponding wavelengths fromabout 10 to about 100 km long.

“Low-frequency” (LF) generally designates the band of frequencies in therange of about 30 kHz to about 300 kHz with wavelengths range from about1 to about 10 km.

“Medium frequency” (MF) generally designates the band of frequenciesfrom about 300 kHz to about 3 MHz with wavelengths from about 1000 toabout 100 m long.

“High frequency” (HF) generally designates the band of frequencies fromabout 3 MHz to about 30 MHz having wavelengths from about 100 m to about10 m long.

“Very high frequency” (VHF) generally designates the band of frequenciesfrom about 30 Hz to about 300 MHz with wavelengths from about 10 m toabout 1 m long.

“Ultra high frequency” (UHF) generally designates the band offrequencies from about 300 MHz to about 3 GHz with weight wavelengthsranging from about 1 m to about 10 cm long.

“Super high frequency” (SHF) generally designates the band offrequencies from about 3 GHz to about 30 GHz with wavelengths rangingfrom about 10 cm to about 1 cm long.

“Extremely high frequency” (EHF) generally designates the band offrequencies from about 30 GHz to about 300 GHz with wavelengths rangingfrom about 1 cm to about 1 mm long.

“Far infrared” (FIR) generally designates a band of frequencies fromabout 300 GHz to about 20 THz with wavelengths ranging from about 1 mmto about 15 μm long.

“Long-wavelength infrared” (LWIR) generally designates a band offrequencies from about 20 THz to about 37 THz with wavelengths rangingfrom about 15 μm to about 8 μm long.

“Mid infrared” (MIR) generally designates a band of frequencies fromabout 37 THz to about 100 THz with wavelengths from about 8 μm to about3 μm long.

“Short wavelength infrared” (SWIR) generally designates a band offrequencies from about 100 THz to about 214 THz with wavelengths fromabout 3 μm to about 1.4 μm long.

“Near-infrared” (NIR) generally designates a band of frequencies fromabout 214 THz to about 400 THz with wavelengths from about 1.4 μm toabout 750 nm long.

“Visible light” generally designates a band of frequencies from about400 THz to about 750 THz with wavelengths from about 750 nm to about 400nm long.

“Near ultraviolet” (NUV) generally designates a band of frequencies fromabout 750 THz to about 1 PHz with wavelengths from about 400 nm to about300 nm long.

“Middle ultraviolet” (MUV) generally designates a band of frequenciesfrom about 1 PHz to about 1.5 PHz with wavelengths from about 300 nm toabout 200 nm long.

“Far ultraviolet” (FUV) generally designates a band of frequencies fromabout 1.5 PHz to about 2.48 PHz with wavelengths from about 200 nm toabout 122 nm long.

“Extreme ultraviolet” (EUV) generally designates a band of frequenciesfrom about 2.48 PHz to about 30 PHz with wavelengths from about 121 nmto about 10 nm long.

“Soft x-rays” (SX) generally designates a band of frequencies from about30 PHz to about 3 EHz with wavelengths from about 10 nm to about 100 pmlong.

“Hard x-rays” (HX) generally designates a band of frequencies from about3 EHz to about 30 EHz with wavelengths from about 100 μm to about 10 pmlong.

“Gamma rays” generally designates a band of frequencies above about 30EHz with wavelengths less than about 10 pm long.

“Electromagnetic Waves” generally refers to waves having a separateelectrical and a magnetic component. The electrical and magneticcomponents of an electromagnetic wave oscillate in phase and are alwaysseparated by a 90 degree angle. Electromagnetic waves can radiate from asource to create electromagnetic radiation capable of passing through amedium or through a vacuum. Electromagnetic waves include wavesoscillating at any frequency in the electromagnetic spectrum including,but not limited to, radio waves, visible and invisible light, X-rays,and gamma-rays.

“Equalizer” generally refers to a device used to reduce distortion in atransmitted signal. An equalizer may reduce distortion based on timesuch as a group delay or may reduce distortion of signal frequency suchas phase delay. An equalizer may also be used to reduce other types ofdistortion such as echoes, frequency selectivity, or frequency shift.

“Frequency Bandwidth” or “Band” generally refers to a contiguous rangeof frequencies defined by an upper and lower frequency. Frequencybandwidth is thus typically expressed as a number of hertz (cycles persecond) representing the difference between the upper frequency and thelower frequency of the band and may or may not include the upper andlower frequencies themselves. A “band” can therefore be defined by agiven frequency bandwidth for a given region and designated withgenerally agreed on terms. For example, the “20 meter band” in theUnited States is assigned the frequency range from 14 MHz to 14.35 MHzthus defining a frequency bandwidth of 0.35 MHz or 350 KHz. In anotherexample, the International Telecommunication Union (ITU) has designatedthe frequency range from 300 Mhz to 3 GHz as the “UHF band”.

“Fiber-optic communication” generally refers to a method of transmittingdata from one place to another by sending pulses of electromagneticenergy through an optical fiber. The transmitted energy may form anelectromagnetic carrier wave that can be modulated to carry data.Fiber-optic communication lines that use optical fiber cables totransmit data can be configured to have a high data bandwidth. Forexample, fiber-optic communication lines may have a high data bandwidthof up to about 15 Tbit/s, about 25 Tbit/s, about 100 Tbit/s, about 1Pbit/s or more. Opto-electronic repeaters may be used along afiber-optic communication line to convert the electromagnetic energyfrom one segment of fiber-optic cable into an electrical signal. Therepeater can retransmit the electrical signal as electromagnetic energyalong another segment of fiber-optic cable at a higher signal strengththan it was received.

“Financial instrument” generally refers to a tradable asset of any kind.General examples include, but are not limited to, cash, evidence of anownership interest in an entity, or a contractual right to receive ordeliver cash or another financial instrument. Specific examples includebonds, bills (e.g. commercial paper and treasury bills), stock, loans,deposits, certificates of deposit, bond futures or options on bondfutures, short-term interest rate futures, stock options, equityfutures, currency futures, interest rate swaps, interest rate caps andfloors, interest rate options, forward rate agreements, stock options,foreign-exchange options, foreign-exchange swaps, currency swaps, or anysort of derivative.

“Fused data stream” generally refers to a combination of at least two ormore separate data transmissions. The data transmissions may come fromany desired source. For example, the data transmission may be in-banddata, out-of-band data, public data, or private data. The fused datastream may be any desired combination of these different datatransmissions.

“Ground” is used more in an electrical/electromagnetic sense andgenerally refers to the Earth's surface including land and bodies ofwater, such as oceans, lakes, and rivers.

“Ground-wave propagation” generally refers to a transmission method inwhich one or more electromagnetic waves are conducted via the boundaryof the ground and atmosphere to travel along ground. The electromagneticwave propagates by interacting with the semi-conductive surface of theearth. In essence, the wave clings to the surfaces so as to follow thecurvature of the earth. Typically, but not always, the electromagneticwave is in the form of a ground or surface wave formed by low-frequencyradio waves.

“Identifier” generally refers to a name that identifies (that is, labelsthe identity of) either a unique thing or a unique class of things,where the “object” or class may be an idea, physical object (or classthereof), or physical substance (or class thereof). The abbreviation“ID” often refers to identity, identification (the process ofidentifying), or an identifier (that is, an instance of identification).An identifier may or may not include words, numbers, letters, symbols,shapes, colors, sounds, or any combination of those.

The words, numbers, letters, or symbols may follow an encoding system(wherein letters, digits, words, or symbols represent ideas or longeridentifiers) or they may simply be arbitrary. When an identifier followsan encoding system, it is often referred to as a code or ID code.Identifiers that do not follow any encoding scheme are often said to bearbitrary IDs because they are arbitrarily assigned without meaning inany other context beyond identifying something.

“In-band data” generally refers to data that is collected from the maindata transmission stream between two communication nodes. Typically,in-band data is the main data transmission sent by the transmittingparty. This data may be collected and analyzed to determine theviability of transmitting data at a certain frequency at the ionosphericconditions during the time of transmission.

“Ionosphere” generally refers to the layer of the Earth's atmospherethat contains a high concentration of ions and free electrons and isable to reflect radio waves. The ionosphere includes the thermosphere aswell as parts of the mesosphere and exosphere. The ionosphere extendsfrom about 25 to about 600 miles (about 40 to 1,000 km) above theearth's surface. The ionosphere includes a number of layers that undergoconsiderable variations in altitude, density, and thickness, dependingamong a number of factors including solar activity, such as sunspots.The various layers of the ionosphere are identified below.

The “D layer” of the ionosphere is the innermost layer that ranges fromabout 25 miles (40 km) to about 55 miles (90 km) above the Earth'ssurface. The layer has the ability to refract signals of lowfrequencies, but it allows high frequency radio signals to pass throughwith some attenuation. The D layer normally, but not in all instances,disappears rapidly after sunset due to rapid recombination of its ions.

The “E layer” of the ionosphere is the middle layer that ranges fromabout 55 miles (90 km) to about 90 miles (145 km) above the Earth'ssurface. The E layer typically has the ability to refract signals withfrequencies higher than the D layer. Depending on the conditions, the Elayer can normally refract frequencies up to 20 MHz. The rate of ionicrecombination in the E layer is somewhat rapid such that after sunset italmost completely disappears by midnight. The E layer can furtherinclude what is termed an “E_(s)-layer” or “sporadic E layer” that isformed by small, thin clouds of intense ionization. The sporadic E layercan reflect radio waves, even frequencies up to 225 MHz, althoughrarely. Sporadic E layers most often form during summer months, and ithas skip distances of around 1,020 miles (1,640 km). With the sporadic Elayer, one hop propagation can be about 560 miles (900 km) to up to1,600 miles (2,500 km), and double hop propagation can be over 2,200miles (3,500 km).

The “F layer” of the ionosphere is the top layer that ranges from about90 (145 km) to 310 miles (500 km) or more above the Earth's surface. Theionization in the F layer is typically quite high and varies widelyduring the day, with the highest ionization occurring usually aroundnoon. During daylight, the F layer separates into two layers, the F₁layer and the F₂ layer. The F₂ layer is outermost layer and, as such, islocated higher than the F₁ layer. Given the atmosphere is rarified atthese altitudes, the recombination of ions occur slowly such that Flayer remains constantly ionized, either day or night such that most(but not all) skywave propagation of radio waves occur in the F layer,thereby facilitating high frequency (HF) or short wave communicationover long distances. For example, the F layers are able to refract highfrequency, long distance transmissions for frequencies up to 30 MHz.

“Latency” generally refers to the time interval between a cause and aneffect in a system. Latency is physically a consequence of the limitedvelocity with which any physical interaction can propagate throughout asystem. Latency is physically a consequence of the limited velocity withwhich any physical interaction can propagate. The speed at which aneffect can propagate through a system is always lower than or equal tothe speed of light. Therefore every physical system that includes somedistance between the cause and the effect will experience some kind oflatency. For example, in a communication link or communications network,latency generally refers to the minimum time it takes for data to passfrom one point to another. Latency with respect to communicationsnetworks may also be characterized as the time it takes energy to movefrom one point along the network to another. With respect to delayscaused by the propagation of electromagnetic energy following aparticular propagation path, latency can be categorized as follows:

“Low Latency” generally refers to a period of time that is less than orabout equal to a propagation time that is 10% greater than the timerequired for light to travel a given propagation path in a vacuum.Expressed as a formula, low latency is defined as follows:

$\begin{matrix}{{{latenc}y_{low}} \leq {\frac{d}{c} \cdot k}} & ( {{Equation}1} )\end{matrix}$

-   -   where:    -   d=distance (miles)    -   c=the speed of light in a vacuum (186,000 miles/sec)    -   k=a scalar constant of 1.1

For example, light can travel 25,000 miles through a vacuum in about0.1344 seconds. A “low latency” communication link carrying data overthis 25,000 mile propagation path would therefore be capable of passingat least some portion of the data over the link in about 0.14784 secondsor less.

“High Latency” generally refers to a period of time that is over 10%greater than the time required for light to travel a given propagationpath in a vacuum. Expressed as a formula, high latency is defined asfollows:

$\begin{matrix}{{{latenc}y_{high}} > {\frac{d}{c} \cdot k}} & ( {{Equation}2} )\end{matrix}$

-   -   where:    -   d=distance (miles)    -   c=the speed of light in a vacuum (186,000 miles/sec)    -   k=a scalar constant of 1.1

For example, light can travel 8,000 miles through a vacuum in about0.04301 seconds. A “high latency” communication link carrying data overthis transmission path would therefore be capable of passing at leastsome portion of the data over the link in about 0.04731 seconds or more.

The “high” and “low” latency of a network may be independent of the databandwidth. Some “high” latency networks may have a high transfer ratethat is higher than a “low” latency network, but this may not always bethe case. Some “low” latency networks may have a data bandwidth thatexceeds the bandwidth of a “high” latency network.

“Maximum Usable Frequency (MUF)” generally refers to the highestfrequency that is returned to the Earth using sky-wave propagation.

“Memory” generally refers to any storage system or device configured toretain data or information. Each memory may include one or more types ofsolid-state electronic memory, magnetic memory, or optical memory, justto name a few. By way of non-limiting example, each memory may includesolid-state electronic Random Access Memory (RAM), SequentiallyAccessible Memory (SAM) (such as the First-In, First-Out (FIFO) varietyor the Last-In-First-Out (LIFO) variety), Programmable Read Only Memory(PROM), Electronically Programmable Read Only Memory (EPROM), orElectrically Erasable Programmable Read Only Memory (EEPROM); an opticaldisc memory (such as a DVD or CD ROM); a magnetically encoded hard disc,floppy disc, tape, or cartridge media; or a combination of any of thesememory types. Also, each memory may be volatile, nonvolatile, or ahybrid combination of volatile and nonvolatile varieties.

“Modulation” generally refers to a process of varying one or moreproperties of a periodic waveform, called a carrier signal, with amodulating signal that represents information to be transmitted. Commonexamples of modulation techniques include, but are not limited to,Frequency Modulation (FM), Amplitude Modulation (AM), Phase—Shift Keying(PSK), Frequency Shift Keying (FSK), Amplitude Shift Keying (Ask), offKeying (Oh Okay) Quadrature Amplitude Modulation (QAM), Continuous PhaseModulation (CPM), Orthogonal Frequency Division Multiplexing (OFD M),Wavelet Modulation, Trellis Coded Modulation (TCM), Direct SequenceSpread Spectrum (DSSS), Troop Spread Spectrum (CSS), and FrequencyHopping Spread Spectrum (FHSS).

“Modulator” generally refers to a device that is configured or adaptedto perform modulation. One example of such a device is a “modem” whichcan perform both modulation and demodulation.

“Non-sky-wave propagation” generally refers to all forms oftransmission, wired and/or wireless, in which the information is nottransmitted by reflecting an electromagnetic wave from the ionosphere.

“Optimum Working Frequency” generally refers to the frequency thatprovides the most consistent communication path via sky-wavepropagation. It can vary over time depending on number of factors, suchas ionospheric conditions and time of day. For transmissions using theF2 layer of the ionosphere the working frequency is generally around 85%of the MUF, and for the E layer, the optimum working frequency willgenerally be near the MUF.

“Optical Fiber” generally refers to an electromagnetic waveguide havingan elongate conduit that includes a substantially transparent mediumthrough which electromagnetic energy travels as it traverses the longaxis of the conduit. Electromagnetic radiation may be maintained withinthe conduit by total internal reflection of the electromagneticradiation as it traverses the conduit. Total internal reflection isgenerally achieved using optical fibers that include a substantiallytransparent core surrounded by a second substantially transparentcladding material with a lower index of refraction than the core.

Optical fibers are generally constructed of dielectric material that isnot electrically conductive but is substantially transparent. Suchmaterials may or may not include any combination of extruded glass suchas silica, fluoride glass, phosphate glass, Chalcogenide glass, orpolymeric material such as various types of plastic, or other suitablematerial and may be configured with any suitable cross-sectional shape,length, or dimension. Examples of electromagnetic energy that may besuccessfully passed through optical fibers include electromagnetic wavesin the near-infrared, mid-infrared, and visible light portion of theelectromagnetic spectrum, although electromagnetic energy of anysuitable frequency may be used.

“Out-of-band data” generally refers to data that is collected from achannel that is independent of the channel through which the main datastream is transmitted. The out-of-band data may be data streams sent byskywave propagation by third parties or may be data streams sent by thetransmitting party along a different channel than the main datatransmission stream. The data collected may include ionospheric data,for example from an ionosonde, or may be general data that is collectedand analyzed to determine the viability of transmitting data at acertain frequency at the current ionospheric conditions.

“Polarization” generally refers to the orientation of the electric field(“E-plane”) of a radiated electromagnetic energy wave with respect tothe Earth's surface and is determined by the physical structure andorientation of the radiating antenna. Polarization can be consideredseparately from an antenna's directionality. Thus, a simple straightwire antenna may have one polarization when mounted abstention thevertically, and a different polarization when mounted substantiallyhorizontally. As a transverse wave, the magnetic field of a radio waveis at right angles to that of the electric field, but by convention,talk of an antenna's “polarization” is understood to refer to thedirection of the electric field.

Reflections generally affect polarization. For radio waves, oneimportant reflector is the ionosphere which can change the wave'spolarization. Thus for signals received via reflection by the ionosphere(a skywave), a consistent polarization cannot be expected. Forline-of-sight communications or ground wave propagation, horizontally orvertically polarized transmissions generally remain in about the samepolarization state at the receiving location. Matching the receivingantenna's polarization to that of the transmitter may be especiallyimportant in ground wave or line of sight propagation but may be lessimportant in skywave propagation.

An antenna's linear polarization is generally along the direction (asviewed from the receiving location) of the antenna's currents when sucha direction can be defined. For instance, a vertical whip antenna orWi-Fi antenna vertically oriented will transmit and receive in thevertical polarization. Antennas with horizontal elements, such as mostrooftop TV antennas, are generally horizontally polarized (becausebroadcast TV usually uses horizontal polarization). Even when theantenna system has a vertical orientation, such as an array ofhorizontal dipole antennas, the polarization is in the horizontaldirection corresponding to the current flow.

Polarization is the sum of the E-plane orientations over time projectedonto an imaginary plane perpendicular to the direction of motion of theradio wave. In the most general case, polarization is elliptical,meaning that the polarization of the radio waves varies over time. Twospecial cases are linear polarization (the ellipse collapses into aline) as we have discussed above, and circular polarization (in whichthe two axes of the ellipse are equal). In linear polarization theelectric field of the radio wave oscillates back and forth along onedirection; this can be affected by the mounting of the antenna butusually the desired direction is either horizontal or verticalpolarization. In circular polarization, the electric field (and magneticfield) of the radio wave rotates At the radio frequency circularlyaround the axis of propagation.

“Private data” generally refers to ionospheric data that is collectedfrom sources that are not available to the general public. Private datamay be historical or current ionospheric data collected by the partythat is performing data transmission, or may be ionospheric data that ispurchased from a third party by the party that is performing datatransmission. Private data may also be high frequency data transmissionssent by skywave propagation that may be collected and analyzed fortransmission properties, such as distortion, that may indicate theviability of a certain transmission frequency.

“Processor” generally refers to one or more electronic componentsconfigured to operate as a single unit configured or programmed toprocess input to generate an output. Alternatively, when of amulti-component form, a processor may have one or more componentslocated remotely relative to the others. One or more components of eachprocessor may be of the electronic variety defining digital circuitry,analog circuitry, or both. In one example, each processor is of aconventional, integrated circuit microprocessor arrangement, such as oneor more PENTIUM, i3, i5 or i7 processors supplied by INTEL Corporationof 2200 Mission College Boulevard, Santa Clara, Calif. 95052, USA.

Another example of a processor is an Application-Specific IntegratedCircuit (ASIC). An ASIC is an Integrated Circuit (IC) customized toperform a specific series of logical operations is controlling thecomputer to perform specific tasks or functions. An ASIC is an exampleof a processor for a special purpose computer, rather than a processorconfigured for general-purpose use. An application-specific integratedcircuit generally is not reprogrammable to perform other functions andmay be programmed once when it is manufactured.

In another example, a processor may be of the “field programmable” type.Such processors may be programmed multiple times “in the field” toperform various specialized or general functions after they aremanufactured. A field-programmable processor may include aField-Programmable Gate Array (FPGA) in an integrated circuit in theprocessor. FPGA may be programmed to perform a specific series ofinstructions which may be retained in nonvolatile memory cells in theFPGA. The FPGA may be configured by a customer or a designer using ahardware description language (HDL). In FPGA may be reprogrammed usinganother computer to reconfigure the FPGA to implement a new set ofcommands or operating instructions. Such an operation may be executed inany suitable means such as by a firmware upgrade to the processorcircuitry.

Just as the concept of a computer is not limited to a single physicaldevice in a single location, so also the concept of a “processor” is notlimited to a single physical logic circuit or package of circuits butincludes one or more such circuits or circuit packages possiblycontained within or across multiple computers in numerous physicallocations. In a virtual computing environment, an unknown number ofphysical processors may be actively processing data, the unknown numbermay automatically change over time as well.

The concept of a “processor” includes a device configured or programmedto make threshold comparisons, rules comparisons, calculations, orperform logical operations applying a rule to data yielding a logicalresult (e.g. “true” or “false”). Processing activities may occur inmultiple single processors on separate servers, on multiple processorsin a single server with separate processors, or on multiple processorsphysically remote from one another in separate computing devices.

“Public data” generally refers to ionospheric data that is freelyavailable to the public or any interested party. Public data may beionosonde data collected and made available by governmental agenciessuch as NASA, the National Oceanic and Atmospheric Administration(NOAA), or any other public entity that collects and distributesionospheric data. Public data may be historic data or real-time data.Public data may also be high frequency data transmissions sent byskywave propagation that may be collected and analyzed for transmissionproperties, such as distortion, that may indicate the viability of acertain transmission frequency.

“Radio” generally refers to electromagnetic radiation in the frequenciesthat occupy the range from 3 kHz to 300 GHz.

“Radio horizon” generally refers the locus of points at which directrays from an antenna are tangential to the ground. The radio horizon canbe approximated by the following equation:d≃√{square root over (2h _(t))}+√{square root over (2h _(r))}  (Equation3)

-   -   where:    -   d=radio horizon (miles)    -   h_(t)=transmitting antenna height (feet)    -   h_(r)=receiving antenna height (feet).

“Remote” generally refers to any physical, logical, or other separationbetween two things. The separation may be relatively large, such asthousands or millions of miles or kilometers, or small such asnanometers or millionths of an inch. Two things “remote” from oneanother may also be logically or physically coupled or connectedtogether.

“Receive” generally refers to accepting something transferred,communicated, conveyed, relayed, dispatched, or forwarded. The conceptmay or may not include the act of listening or waiting for something toarrive from a transmitting entity. For example, a transmission may bereceived without knowledge as to who or what transmitted it. Likewisethe transmission may be sent with or without knowledge of who or what isreceiving it. To “receive” may include, but is not limited to, the actof capturing or obtaining electromagnetic energy at any suitablefrequency in the electromagnetic spectrum. Receiving may occur bysensing electromagnetic radiation. Sensing electromagnetic radiation mayinvolve detecting energy waves moving through or from a medium such as awire or optical fiber. Receiving includes receiving digital signalswhich may define various types of analog or binary data such as signals,datagrams, packets and the like.

“Receiving Station” generally refers to a receiving device, or to alocation facility having multiple devices configured to receiveelectromagnetic energy. A receiving station may be configured to receivefrom a particular transmitting entity, or from any transmitting entityregardless of whether the transmitting entity is identifiable in advanceof receiving the transmission.

“Reflection point” generally refers to the location in the ionosphere atwhich a radio wave is refracted by the ionosphere so that it begins totravel back to the surface of the earth rather than further into theatmosphere.

“Sensor” generally refers to any device that detects or measures aphysical property. The physical property that is measured may be anatmospheric condition, but this is not required. For example, a sensormay measure atmospheric conditions, such as ionospheric height. A sensormay also collect data related to temperature, wind speed, lightning, orany of a number of other weather related parameters. A sensor may belimited to the measurement of a single physical property or may becapable of measuring several different physical properties.

“Skip distance” generally refers to the minimum distance from atransmitter to where a wave from sky-wave propagation can be returned tothe Earth. To put it another way, the skip distance is the minimumdistance that occurs at the critical angle for sky-wave propagation.

“Skip zone” or “quiet zone” generally refers to is an area between thelocation where a ground wave from ground wave propagation is completelydissipated and the location where the first sky wave returns using skywave propagation. In the skip zone, no signal for a given transmissioncan be received.

“Satellite communication” or “satellite propagation” generally refers totransmitting one or more electromagnetic signals to a satellite which inturn reflects and/or retransmits the signal to another satellite orstation.

“Signal” generally refers to a detectable physical quantity and/orimpulse by which information and/or instructions can be transmitted. Thephysical properties being measured to determine the signal for examplecan include sound, light, and/or electromagnetic radiation, to name justa few. For instance, electrical voltage and/or electrical current can bemeasured to determine an electrical signal transmitted by a wire, andchanges to electromagnetic field amplitude, frequency, phase, intensity,and/or strength can also be used to send and detect a signal wirelessly.

“Size” generally refers to the extent of something; a thing's overalldimensions or magnitude; how big something is. For physical objects,size may be used to describe relative terms such as large or larger,high or higher, low or lower, small or smaller, and the like. Size ofphysical objects may also be given in fixed units such as a specificwidth, length, height, distance, volume, and the like expressed in anysuitable units.

For data transfer, size may be used to indicate a relative or fixedquantity of data being manipulated, addressed, transmitted, received, orprocessed as a logical or physical unit. Size may be used in conjunctionwith the amount of data in a data collection, data set, data file, orother such logical unit. For example, a data collection or data file maybe characterized as having a “size” of 35 Mbytes, or a communicationlink may be characterized as having a data bandwidth with a “size” of1000 bits per second.

“Sky-wave propagation” refers generally to a transmission method inwhich one or more electromagnetic-waves radiated from an antenna arerefracted from the ionosphere back to the ground. Sky-wave propagationfurther includes tropospheric scatter transmissions. In one form, askipping method can be used in which the waves refracted from theionosphere are reflected by the ground back up to the ionosphere. Thisskipping can occur more than once.

“Space-wave propagation” or sometimes referred to as “direct wavepropagation” or “line-of-sight propagation” generally refers to atransmission method in which one or more electromagnetic waves aretransmitted between antennas that are generally visible to one another.The transmission can occur via direct and/or ground reflected spacewaves. Generally speaking, the antenna height and curvature of the earthare limiting factors for the transmission distances for space-wavepropagation. The actual radio horizon for a direct line of sight islarger than the visible or geometric line of sight due to diffractioneffects; that is, the radio horizon is about ⅘ greater than thegeometric line of sight.

“Spread spectrum” generally refers to a transmission method thatincludes sending a portion of a transmitted signal over multiplefrequencies. The transmission over multiple frequencies may occursimultaneously by sending a portion of the signal on variousfrequencies. In this example, a receiver must listen to all frequenciessimultaneously in order to reassemble the transmitted signal. Thetransmission may also be spread over multiple frequencies by “hopping”signals. A signal hopping scenario includes transmitting the signal forsome period of time over a first frequency, switching to transmit thesignal over a second frequency for a second period of time, beforeswitching to a third frequency for a third period of time, and so forth.The receiver and transmitter must be synchronized in order to switchfrequencies together. This process of “hopping” frequencies may beimplemented in a frequency-hopping pattern that may change over time(e.g. every hour, every 24 hours, and the like).

“Stratosphere” generally refers to a layer of the Earth's atmosphereextending from the troposphere to about 25 to 35 miles above the earthsurface.

“Transfer Rate” generally refers to the rate at which a something ismoved from one physical or logical location to another. In the case of acommunication link or communication network, a transfer rate may becharacterized as the rate of data transfer over the link or network.Such a transfer rate may be expressed in “bits per second” and may belimited by the maximum data bandwidth for a given network orcommunication link used to carry out a transfer of data.

“Transmission frequency model” generally refers to a method ofdetermining a suitable frequency for data transmission along aconsistent communication path via skywave propagation. The transmissionfrequency model may be used to determine a suitable frequency fortransmission in real time and/or may be used to predict future suitablefrequencies as well as when to switch frequency of data transmission. Atransmission frequency model may accept various types of data as aninput, for example transmitted data streams, environmental data,historical data, and any other desired types of data for determining atransmission frequency. In some instances, a transmission frequencymodel may be a computer program and stored in computer memory andoperable using a computer processor.

“Transmission line” generally refers to a specialized physical structureor series of structures designed to carry electromagnetic energy fromone location to another, usually without radiating the electromagneticenergy through free space. A transmission line operates to retain andtransfer electromagnetic energy from one location to another whileminimizing latency and power losses incurred as the electromagneticenergy passes through the structures in the transmission line.

Examples of transmission lines that may be used in communicating radiowaves include twin lead, coaxial cable, microstrip, strip line,twisted-pair, star quad, lecher lines, various types of waveguide, or asimple single wire line. Other types of transmission lines such asoptical fibers may be used for carrying higher frequency electromagneticradiation such as visible or invisible light.

“Transmission Path” or “Propagation Path” generally refers to path takenby electromagnetic energy passing through space or through a medium.This can include transmissions through a transmission line. In thiscase, the transmission path is defined by, follows, is contained within,passes through, or generally includes the transmission line. Atransmission or propagation path need not be defined by a transmissionline. A propagation or transmission path can be defined byelectromagnetic energy moving through free space or through theatmosphere such as in skywave, ground wave, line-of-site, or other formsof propagation. In that case, the transmission path can be characterizedas any path along which the electromagnetic energy passes as it is movesfrom the transmitter to the receiver, including any skip, bounce,scatter, or other variations in the direction of the transmitted energy.

“Transmission Station” generally refers to a transmitting device, or toa location or facility having multiple devices configured to transmitelectromagnetic energy. A transmission station may be configured totransmit to a particular receiving entity, to any entity configured toreceive transmission, or any combination thereof.

“Transmit” generally refers to causing something to be transferred,communicated, conveyed, relayed, dispatched, or forwarded. The conceptmay or may not include the act of conveying something from atransmitting entity to a receiving entity. For example, a transmissionmay be received without knowledge as to who or what transmitted it.Likewise the transmission may be sent with or without knowledge of whoor what is receiving it. To “transmit” may include, but is not limitedto, the act of sending or broadcasting electromagnetic energy at anysuitable frequency in the electromagnetic spectrum. Transmissions mayinclude digital signals which may define various types of binary datasuch as datagrams, packets and the like. A transmission may also includeanalog signals.

“Triggering Data” generally refers to data that includes triggeringinformation identifying one or more commands to execute. The triggeringdata and the command data may occur together in a single transmission ormay be transmitted separately along a single or multiple communicationlinks.

“Troposphere” generally refers to the lowest portion of the Earth'satmosphere. The troposphere extends about 11 miles above the surface ofthe earth in the mid-latitudes, up to 12 miles in the tropics, and about4.3 miles in winter at the poles.

“Tropospheric scatter transmission” generally refers to a form ofsky-wave propagation in which one or more electromagnetic waves, such asradio waves, are aimed at the troposphere. While not certain as to itscause, a small amount of energy of the waves is scattered forwards to areceiving antenna. Due to severe fading problems, diversity receptiontechniques (e.g., space, frequency, and/or angle diversity) aretypically used.

“Unmanned Aerial Vehicle (UAV)” or “Drone” generally refers to anaircraft without a human pilot aboard. A UAV may be operated via aground-based controller by a human and/or can be operated autonomouslyusing an onboard computer. A UAV may also operate without human orcomputer aided control and may be directed by environmental factors suchas wind speed and wind direction. A drone may be powered or unpowered. AUAV may include a combustion type engine (e.g., a turbine) that ispowered with fuel and/or may use alternative power sources such aselectrical motors that are powered with solar cells and/or a battery. Byway of non-limiting examples, the UAV can include balloons, dirigibleairships, blimps, airplanes, helicopters, quadcopters, gliders, and/orother types of aircraft.

“Wave Guide” generally refers to a transmission line configured toguides waves such as electromagnetic waves occurring at any frequencyalong the electromagnetic spectrum. Examples include any arrangement ofconductive or insulative material configured to transfer lower frequencyelectromagnetic radiation ranging along the electromagnetic spectrumfrom extremely low frequency to extremely high frequency waves. Othersspecific examples include optical fibers guiding high-frequency light orhollow conductive metal pipe used to carry high-frequency radio waves,particularly microwaves.

It should be noted that the singular forms “a”, “an”, “the”, and thelike as used in the description and/or the claims include the pluralforms unless expressly discussed otherwise. For example, if thespecification and/or claims refer to “a device” or “the device”, itincludes one or more of such devices.

It should be noted that directional terms, such as “up”, “down”, “top”“bottom”, “fore”, “aft”, “lateral”, “longitudinal”, “radial”,“circumferential”, etc., are used herein solely for the convenience ofthe reader in order to aid in the reader's understanding of theillustrated embodiments, and it is not the intent that the use of thesedirectional terms in any manner limit the described, illustrated, and/orclaimed features to a specific direction and/or orientation.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges, equivalents, and modifications that come within the spirit ofthe inventions defined by following claims are desired to be protected.All publications, patents, and patent applications cited in thisspecification are herein incorporated by reference as if each individualpublication, patent, or patent application were specifically andindividually indicated to be incorporated by reference and set forth inits entirety herein.

What is claimed is:
 1. A system comprising: a transmitter including atransmitter clock device, wherein the transmitter is configured toswitch between transmission of a digital signal and an analog signal; areceiver including a receiver clock device, wherein the receiver isconfigured to receive the digital signal and the analog signaltransmitted from the transmitter; wherein the transmitter is configuredto transmit a clock synchronization signal to the receiver, wherein theclock synchronization signal is configured to synchronize thetransmitter clock device and the receiver clock device; wherein theclock synchronization signal is embedded in the analog signal so thatthe clock synchronization signal is transmitted with the analog signal;wherein the transmitter clock device and the receiver clock device aresynchronized by the clock synchronization signal without a delay whenthe transmitter switches from transmitting the analog signal to thedigital signal; and wherein clock synchronization signal is transmittedwith the analog signal only for the time needed to establishsynchronization before the transmitter switches from the analog signalto the digital signal.
 2. The system of claim 1, wherein the analogsignal is an Amplitude Modulation (AM) signal.
 3. The system of claim 1,wherein the clock synchronization signal is a phase modulated signal. 4.The system of claim 1, wherein the analog signal is used for stationidentification.
 5. The system of claim 1, wherein the analog signal isan audio broadcast.
 6. The system of claim 1, wherein the digital signaltransmits financial trading data.
 7. The system of claim 1, wherein theanalog signal is transmitted wirelessly by skywave propagation.
 8. Thesystem of claim 1, wherein the transmitter clock device and the receiverclock device are set based on time data received from a globalpositioning system.
 9. A method comprising: embedding a clocksynchronization signal in an analog signal wherein the clocksynchronization signal is configured to synchronize a transmitter clockand a receiver clock; transmitting the analog signal and the embeddedclock synchronization signal from a transmitter to a receiver; switchingthe transmission from the transmitter from the analog signal to adigital signal; wherein the transmitter clock and the receiver clock aresynchronized by the clock synchronization signal without a delay whenthe transmitter switches from transmitting the analog signal to thedigital signal; and wherein the clock synchronization signal istransmitted with the analog signal only for the time needed to establishsynchronization before the transmitter switches from the analog signalto the digital signal.
 10. The method of claim 9, wherein the analogsignal is an Amplitude Modulation (AM) signal.
 11. The method of claim9, wherein the clock synchronization signal is a phase modulated signal.12. The method of claim 9, wherein the analog signal is used for stationidentification.
 13. The method of claim 9, wherein the analog signal isan audio broadcast.
 14. The method of claim 9, wherein the digitalsignal transmits financial trading data.
 15. The method of claim 9,wherein the analog signal is transmitted wirelessly by skywavepropagation.
 16. The method of claim 9, wherein the transmitter clockand the receiver clock are set based on time data received from a globalpositioning system.